Sound processing and stimulation systems and methods for use with cochlear implant devices

ABSTRACT

Sound processing strategies for use with cochlear implant systems utilizing simultaneous stimulation of electrodes are provided. The strategies include computing a frequency spectrum of a signal representative of sound, arranging the spectrum into channels and assigning a subset of electrodes to each channel. Each subset is stimulated so as to stimulate a virtual electrode positioned at a location on the cochlea that corresponds to the frequency at which a spectral peak is located within an assigned channel. The strategies also derive a carrier for a channel having a frequency that may relate to the stimulation frequency so that temporal information is presented. In order to fit these strategies, a group of electrodes is selected and the portion of the current that would otherwise be applied to electrode(s) having a partner electrode in the group is applied to the partner electrode.

BACKGROUND OF THE INVENTION

The present invention relates to implantable stimulation devices, andmore particularly, to sound processing strategies for use with cochlearimplant systems that utilize simultaneous stimulation of electrodes.

Hearing loss, which may be due to many different causes, is generally oftwo types: conductive and sensorineural. Conductive hearing loss occurswhen the normal mechanical pathways for sound to reach the hair cells inthe cochlea are impeded. Conductive hearing loss may often be helped byuse of conventional hearing aids, which amplify sound so that acousticinformation reaches the cochlea and the hair cells. Sensorineuralhearing loss, on the other hand, is usually due to the absence orimpairment of the hair cells which are needed to transduce acousticsignals in the cochlea into nerve impulses that are sent to the auditorynerve. People suffering from severe sensorineural hearing loss areusually unable to derive any benefit from conventional hearing aidsystems because their mechanisms for transducing sound energy intoauditory nerve impulses are non-existent or have been severely damaged.

Cochlear implant technology seeks to overcome sensorineural hearing lossby bypassing the hair cells in the cochlea and presenting electricalstimulation to the auditory nerve directly, leading to the perception ofsound in the brain and at least partial restoration of hearing. Indeed,cochlear implant technology may be used to bypass the outer, middle andinner ears. Cochlear implant systems that utilize such technology havebeen successfully used to restore hearing in sensorineural deafpatients.

Generally, a cochlear implant system includes an external portion and animplanted portion that are separated by a skin barrier. The externalportion usually includes a power source, a microphone and a signalprocessing device, whereas the implanted portion usually includes astimulation device and an electrode array. The power source suppliespower to the system. Sound enters the system through the microphonewhich delivers it to the signal processing device as an electricalsignal. The signal processing device processes the signal and transmitsit to the stimulation device through the skin barrier. The stimulationdevice uses the received signal to stimulate electrodes in the electrodearray that is implanted into the cochlea. The electrodes in the arraytransmit electrical stimuli to the nerve cells that emanate from thecochlea and that are part of the auditory nerve. These nerve cells arearranged in an orderly tonotopic sequence, from high frequencies nearthe initial (basal) end of the cochlear coil to progressively lowerfrequencies towards the inner end of the coil (apex). Nerve cellsemanating from the various regions of the cochlea are associated withthe frequencies that most efficiently stimulate those regions, and thebrain, which receives neural impulses from the auditory nerve, mapsthose frequencies in accord with this association.

Conventional cochlear implants separate sound signals into a number ofparallel channels of information, each representing the intensity of anarrow band of frequencies within the acoustic spectrum. Ideally, eachchannel of information would be conveyed selectively to the subset ofnerve cells located along the cochlea that would have normallytransmitted information about that frequency band to the brain. Thiswould require placing the electrode array along the entire length of thecochlear ducts, which is surgically impractical. Instead, the electrodearray is typically inserted into the scala tympani, one of the threeparallel ducts that make up the spiral shape of the cochlea. The arrayof linearly arranged electrodes is inserted such that the electrodeclosest to the basal end of the coil is associated with the highestfrequency band and the electrode closest to apex is associated with thelowest frequency band. Each location along the implanted length of thecochlea may be mapped to a corresponding frequency, thereby yielding afrequency-to-location table for the electrode array. The foregoingillustrates the relationship between frequency and physical location inthe cochlea—i.e., the cochlear frequency/location correspondence.

The performance of a cochlear implant system is limited mainly by theamount of information that can be delivered by electrical stimulation tothe patient, which, in turn, is limited by the number of electrodes inthe implant. The number of electrodes that can be used is limited by thesize of the scala tympani and the distance or spatial separation betweenelectrodes. While the size of the scala tympani presents an anatomicallimitation, it is possible to reduce the distance between electrodes.However, reducing such spatial separation increases electrodeinteraction and interference, which could have undesirable effects. Itis however possible to use such effects to deliver additional spectralinformation in a suitable manner.

Recent studies have shown that simultaneously stimulating two adjacentelectrodes in such systems results in patients perceiving a pitch thatis between the two pitches perceived when each electrode is stimulatedindividually. Moreover, as the stimulation current is changed from beingentirely applied to the first electrode to the second electrode, pitchsensation changes from the pitch associated with the first electrode tothe pitch associated with the second electrode in a continuous fashion.This is because the electric field resulting from stimulating one of theelectrodes is likely to be superposed to that resulting from stimulatingthe other electrode. The superposed electric fields are centered arounda virtual electrode that lies between the two adjacent electrodes.Furthermore, the perceived loudness stays roughly constant so long asthe sum of the currents applied to the electrode pair stays roughlyconstant. Thus, it is possible to stimulate a virtual electrode locatedbetween adjacent electrodes by simultaneously stimulating theseelectrodes using relative current weights, whereby the frequency bandassociated with the virtual electrode corresponds to one that liesbetween the frequency bands associated with the individual electrodes.

The challenge lies in utilizing a sound processing strategy that makesuse of such virtual electrodes. Although separate channels could beassigned to individual electrodes and virtual electrodes, such aprocessing scheme limits the number of virtual electrodes that may bestimulated unless the number of channels is significantly increased.Increasing the number of channels, however, demands additionalprocessing capabilities which may result in delays or may require morepower and more complicated circuitry.

Frequencies that may be associated with optimal virtual electrodes mayinstead be estimated. One example of a known system that estimatesfrequencies using conventional methods, such as calculating theinstantaneous frequency along with phase angle and magnitude values, isdescribed in U.S. Pat. No. 6,480,820. However, such methods result ininconsistent representations of spectral sound information. This isbecause adding sound components to the sound stimulus may result in adrastic and disproportional shift in the location on the electrode arrayto be stimulated. Thus, there is a need for improved frequencycomputation for presentation of spectral information in sound processingstrategies that can be used with cochlear implant systems that utilizesimultaneous stimulation of several electrodes.

When a cochlear implant is provided to a patient, it is necessary toinitially fit the system in order for it to better perform its intendedfunction of helping a patient to sense sound at appropriate levels. Acommon method of fitting involves presenting a known sound stimulus to apatient while a subset of the electrode array is activated and adjustingthe level of corresponding electrical current applied to the array suchthat the sound perceived by the patient is of appropriate loudness. Inapplying such a method, it is assumed that the perceived loudness willnot be affected by the activity of adjacent electrodes once the fullarray is activated, including electrodes that were previouslydeactivated. However, when adjacent electrodes are stimulatedsimultaneously, the resulting electric fields are likely to besuperposed thereby affecting the loudness perceived by the patient, asmentioned above. Thus, there is a need for fitting sound processingstrategies used with cochlear implant systems that utilize simultaneousstimulation of several electrodes.

In addition to accounting for spectral information and loudness, acochlear implant system should preserve, as much as possible, temporalinformation that is key to differentiate various sounds. Presenting finetemporal information is critical for the perception of overall soundquality, clarity, speech and music. There is therefore also a need forimproved time detection for presentation of temporal information insound processing strategies used with the cochlear implant systemsdescribed above.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providingsystems and methods that can be used with cochlear implant devices thatutilize simultaneous stimulation of several electrodes. A cochlearimplant system having sound processing circuitry coupled to an electrodearray may be provided. The sound processing circuitry may be adapted tocompute a frequency spectrum of a signal representative of sound andarrange the frequency spectrum into a plurality of channels such thateach channel corresponds to a range of frequencies that lie within thefrequency spectrum. For example, FFT circuitry may perform a DiscreteFourier Transform on the signal in order to compute its frequencyspectrum.

The electrode array may be inserted into the cochlea such that a subsetof electrodes is associated with at least one of the plurality ofchannels. A stream of pulse sets may be simultaneously applied to theelectrodes in the subset so as to stimulate a virtual electrodepositioned at a location on the cochlea that corresponds to astimulation frequency computed for a particular channel. An improvedcomputation of the stimulation frequency may be calculated or estimatedusing the sound processing circuitry as the frequency at which aspectral peak is located within the range of frequencies thatcorresponds to the channel. For example, such a computation may beimplemented using a peak locator through energy computation and functionfitting or estimation. Different subsets of electrodes may be associatedwith the plurality of channels such that the electrode array may be usedto stimulate the auditory nerve at computed stimulation frequencies thatrange over the computed spectrum.

The stream of pulse sets applied to each subset of electrodes may bederived from a current that may be modified based partly on an envelopecomputed by the sound processing circuitry. To do so, the processingcircuitry may divide at least one of the channels into a plurality ofsub-channels having smaller ranges of frequencies and compute a squareroot of a sum of the squared Hilbert envelopes for each of the pluralityof sub-channels. Such a process may be applied to, for example, thechannel assigned to the largest bandwidth in the computed spectrum.

A method for fitting such a sound processing strategy that utilizessimultaneous stimulation of a subset of electrodes may be described inthe following. A fitting group that includes at least one electrode maybe selected and all other electrodes may be disabled. The portion of thecurrent that would otherwise be applied to an electrode that is not partof the fitting group and that has a partner electrode in the fittinggroup is applied instead to the partner electrode in the fitting group.A partner electrode may be one associated with an electric field that issuperposed to that of the electrode that is not part of the fittinggroup so as to stimulate a virtual electrode positioned at a locationthat lies between the location of the two electrodes. Whether a partnerelectrode in the fitting group exists may be determined upon applying aknown stimulus to the electrode array.

The sound processing circuitry may also be adapted to derive a waveformhaving a frequency that is related to a computed frequency for eachchannel. The computed frequency may correspond to the stimulationfrequency or may be calculated using more conventional methods, such asdetermining the instantaneous frequency. For example, the waveform mayhave a frequency that is equal to the computed frequency and may have amodulation depth that decreases as the computed frequency increases.Alternatively, the waveform may have a frequency that is proportional tothe computed frequency. Such a waveform may be used as a carrier forpresenting temporal information in each channel. The stream of pulsesets applied to each subset of electrodes may be derived from a currentthat may be modified based partly on such a carrier. Deriving such acarrier allows for improved time detection for presenting such temporalinformation.

It is therefore a feature of the invention to provide improved frequencycomputation for presenting spectral information in sound processingstrategies that can be used with cochlear implant systems that utilizesimultaneous stimulation of several electrodes.

It is another feature of the invention to provide fitting soundprocessing strategies for such cochlear implant systems.

It is yet another feature of the invention to provide improved timedetection for presenting temporal information in sound processingstrategies that can be used with such cochlear implant systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the inventionwill be apparent upon consideration of the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like reference characters refer to like parts throughout, and inwhich:

FIG. 1 shows several main components of a cochlear implant system inwhich sound processing according to the principles of the presentinvention may be used;

FIG. 2 shows a spectral representation of a sound signal;

FIG. 3 shows an impulse train approximating the representation of FIG. 2according to the principles of the present invention;

FIG. 4 shows a schematic for processing of sound according to theprinciples of the present invention;

FIG. 5 shows a block diagram of sound processing circuitry according tothe principles of the present invention;

FIG. 6 shows a flow diagram for processing sound according to theprinciples of the present invention;

FIG. 7 shows another flow diagram for processing sound according to theprinciples of the present invention;

FIG. 8 shows a flow diagram for fitting sound processing strategiesaccording to the principles of the present invention;

FIG. 9 shows a block diagram of an alternative embodiment of soundprocessing circuitry according to the principles of the presentinvention;

FIG. 10 shows yet another flow diagram for processing sound according tothe principles of the present invention;

FIG. 11 shows an alternative embodiment of yet another flow diagram forprocessing sound according to the principles of the present invention;and

FIG. 12 shows a block diagram of another alternative embodiment of soundprocessing circuitry according to the principles of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a representative block diagram of a cochlear implant system30 that may be used in accordance with the principles of the presentinvention. Cochlear implant system 30 may include unit 32, portion 34,device 14 and array 16. Unit 32 and portion 34 may be coupled throughcable 33 and may be external portions of cochlear implant system 30.Device 14 and array 16, on the other hand, may be implanted into apatient. The external and internal portions may be separated by barrier18, such as a layer of skin. Alternatively, cochlear system 30 may be afully implantable system whereby devices performing the functions ofunit 32 and portion 34 are internal.

Unit 32 may be a behind the ear (“BTE”) unit that may be designed to beworn behind the ear of its user. Unit 32 may include a power source, amicrophone, telemetry transmitter circuitry, as well as other soundprocessing components including those of the present invention.Alternatively, unit 32 may be a portable speech processor (PSP) that mayworn anywhere on the user. In such a situation, unit 32, may include apower source and sound processing components whereas portion 34 mayinclude a microphone. Portion 34 may be a headpiece that houses anantenna coil. Device 14 may include an implantable receiver circuit forgenerating stimulation currents and a cochlear stimulator (“ICS”) forselectively stimulating electrode array 16. Electrode array 16 mayinclude several linearly aligned electrodes 66 (not shown in FIG. 1 butshown in FIGS. 3-5 and 9) and may be inserted within a user's cochlea.

In operation, sound may enter system 30 through the microphone in unit32 which may deliver it to the sound processing components as anelectrical signal. The sound processing components may process thesignal and deliver it via cable 33 to portion 34. Portion 34 may in turntransmit the processed signal through telemetry transmitter circuitry toan implantable receiver circuit in device 14 through barrier 18. Device14 may use the received signal to stimulate the electrodes in electrodearray 16 in order to stimulate the user's auditory nerve. More thoroughdescriptions of certain aspects of elements 32, 33, 14 and 16 may befound, for example, in U.S. Pat. Nos. 4,819,647, 5,603,726, 5,603,726,5,776,172, 6,129,753, 6,181,969, 6,219,580, 6, 289,247, 6,308,101, andU.S. patent application Ser. No. 11/058,848, filed Feb. 15, 2005, whichare incorporated herein by reference in their entireties.

FIG. 2 shows a portional representation of a sound signal that may entersystem 30 of FIG. 1. The frequency domain representation may have, forexample, spectrum 26 as shown in FIG. 2. FIG. 3 shows a correspondingrepresentation of the sound signal shown in FIG. 2 based on a preferredembodiment of a stimulation strategy according to the present invention.More specifically, simultaneous stimulation of groups of electrodes 66in electrode array 16 may generate an impulse train response at selectfrequencies that relatively mimics spectrum 26. Such a strategy takesadvantage of the cochlear frequency/location correspondence describedabove. Such a strategy also takes advantage of the recent showings thatsimultaneous stimulation of adjacent electrodes results in virtualelectrode stimulation, also as described above.

For example, stimulating the first pair of electrodes 66 with a firstcurrent level split between the pair, while assigning more relativeweight—i.e., a higher portion of a current—to second electrode 66 thanfirst electrode 66, may result in generating first impulse 35 at aparticular frequency. First impulse 35 may correspond to a stimulus of avirtual electrode at a location corresponding to that frequency with afirst current level.

Similarly, stimulating the second pair of electrodes 66 with a secondcurrent level split between the pair, while assigning more relativeweight to second electrode 66 than third electrode 66, may result ingenerating second impulse 35 at a particular frequency. Second impulse35 may correspond to a stimulus of a virtual electrode at a locationcorresponding to that frequency with the second current level.

Stimulating the third pair of electrodes 66 with a third current levelsplit between the pair, while assigning more relative weight to fourthelectrode 66 than third electrode 66, may result in generating thirdimpulse 35 at a particular frequency. Third impulse 35 may correspond toa stimulus of a virtual electrode at a location corresponding to thatfrequency with the third current level.

Stimulating the fourth pair of electrodes 66 with a fourth current levelsplit between the pair, while assigning more relative weight to fourthelectrode 66 than fifth electrode 66, may result in generating fourthimpulse 35 at a particular frequency. Fourth impulse 35 may correspondto a stimulus of a virtual electrode at a location corresponding to thatfrequency with the fourth current level.

The foregoing includes examples of simultaneous stimulation of pairs ofadjacent electrodes. Although the following discussion and correspondingdrawings relate to simultaneous stimulation of pairs of adjacentelectrodes, the present invention is not limited thereto. For example,two or more electrodes that may or may not be adjacent to one anothermay be simultaneously stimulated in order to stimulate a virtualelectrode at a desired frequency/location.

FIGS. 4-7 and 9-12 show circuitry and processes that implement soundprocessing strategies for computing, among other things, stimulationfrequencies—i.e., the frequencies that may be associated with optimalvirtual electrodes—according to some embodiments of the presentinvention. Stimulating an optimal virtual electrode at a location alongthe electrode array may be achieved by simultaneously stimulatingsurrounding electrodes. The circuitry and processes shown in FIGS. 4-7and 9-12 may be implemented by electronic circuitry that is mostly partof the external portions of cochlear implant system 30 of FIG. 1, suchas unit 32. For example, unit 32 may have sound processing circuitrythat includes Fast Fourier Transform (“FFT”) circuitry 42, envelopedetector 52, peak locator 54 and navigator 56 shown in FIGS. 4, 5, 9 and12. Moreover, in some embodiments of the present invention, the soundprocessing circuitry in unit 32 may also include other circuitry, suchas carrier synthesizer 90 shown in FIGS. 9 and 12, frequency estimator120 shown in FIG. 12. Furthermore, unit 32 may include other circuitrysuch as mapper 58 of FIGS. 5, 9 and 12 as well as an automatic gaincontrol (“AGC”) circuit (not shown) and at least one buffer (not shown).Alternatively, unit 32 or any other portion of cochlear system 30 mayinclude a subset of the components of FIGS. 4, 5, 9 and 12, while theinternal portion includes at least some of the remaining components. Inalternative embodiments of the present invention, the circuitry andprocesses shown in FIGS. 4-7 and 9-12 may be implemented by softwarethat may be run using cochlear implant system 30.

As mentioned above, sound may enter system 30 through the microphone inunit 32. The microphone may provide an electrical signal that isrepresentative of the sound. The resulting electrical signal may besampled and passed through the AGC circuit, among other things, beforebeing buffered and windowed. The signal may be sampled at a rate r thatmay be proportional to the Nyquist rate. Moreover, sampling rate r maybe selected to be fast enough to allow for proper reconstruction of thetemporal details of the signal. For example, sampling rate r may be setat 17400 Hz or any other appropriate rate. The AGC circuit, which ispreferably a dual-action and programmable circuit, may equalize andcompress the dynamic range of the electrical signal, thereby suppressingdistortion while maintaining fidelity. The output of the AGC circuit maythen be placed into the buffer. The buffer preferably has a length mthat corresponds to the length of the FFT which, as discussed below, isperformed by FFT circuitry 42 in order to compute the frequency spectrumof the signal at a predefined rate. This predefined rate may be relatedto the envelope update rate which is discussed below.

The number of new samples that have been placed in the buffer since thelast frequency spectrum computation may be tracked. If this numberreaches a threshold value, the signal may be windowed and a new FFTcomputation may be initiated. The threshold value preferably relates tothe rate at which the frequency spectrum is computed. For example, ifthe FFT for the signal is computed every 10 samples, then the thresholdvalue may be set at 10 samples. The signal may be windowed in order tosuppress glitches and avoid potential broadening of the frequencyspectrum. During the windowing operation, each sample in the buffer maybe multiplied by a predetermined weight and stored into another buffer.Preferably, if w[j] refers to the window function that is applied, ands[t] refers to the output of the AGC circuit, then:s _(w) [j]=s[t−(m−1)+j]·w[j]

The window function that is applied may be a Hamming window, a Hanningwindow, a Blackman window, a Kaiser window, any combination, such as aweighted sum, of the same or any other suitable window function. Thewindow function typically has a peak at a main lobe about which thewindow is symmetric, and tapers on both sides thereby forming smallerside lobes. Preferably, the window function coefficients are chosen bytrading off the bandwidth of function's main lobe and the height of itsside lobes. For example, the window function may be chosen as an averagebetween a Hanning window and a Blackman window, thereby resulting inside lobes that are approximately 45 dB below the function's peak.

The frequency spectrum of the signal that may have been buffered andwindowed may be computed by through a time-to-frequency domaintransformation. That may be accomplished by computing the DiscreteFourier Transform of the signal. The Discrete Fourier Transform may becomputed through an FFT algorithm. Referring to FIG. 4, FFT circuitry 42may implement such an algorithm, thereby computing the Discrete FourierTransform of the signal as an m point FFT. The FFT may be chosen to beof any appropriate length m—i.e., number of samples—such as 256 samples.The frequency spectrum computation produces a representation of thesignal that is broken down into frequency bins. Preferably, if i_(bin)refers to the bin index—i.e., i_(bin)=0, . . . , (m−1)—and F[i_(bin),t]refers to the Discrete Fourier Transform computed by FFT circuitry 42,while IF[i_(bin),t] refers to the inverse Discrete Fourier Transform,then:

$\begin{matrix}{{F\left\lbrack {i_{bin},t} \right\rbrack} = {{1/m}{\sum\limits_{i = 0}^{m - 1}\;{{s_{w}\lbrack i\rbrack} \cdot {\cos\left( {2\;{\pi \cdot \frac{i_{bin}}{m} \cdot i}} \right)}}}}} \\{{{IF}\left\lbrack {i_{bin},t} \right\rbrack} = {{1/m}{\sum\limits_{i = 0}^{m - 1}\;{{s_{w}\lbrack i\rbrack} \cdot {\sin\left( {2\;{\pi \cdot \frac{i_{bin}}{m} \cdot i}} \right)}}}}}\end{matrix}$

The resulting frequency bins may be organized into a number of channelssuch that the frequency band associated with each channel includes a setof consecutive bins. Each channel may therefore correspond to a range offrequencies that lie within the computed frequency spectrum. The numberof channels may depend on, among other things, the number of electrodesin the electrode array and the desired level of spectral resolution. Forexample, a stimulation strategy that uses an N-electrode array may lenditself to dedicating a single channel to each unique pair of electrodes,thereby yielding a total of N−1 channels, as shown in FIG. 4. In thisexample, channel 1 may be associated with the lowest frequency bins andmay be dedicated to first and second electrodes 66, assuming the firstelectrode to be the one that is closest to the apex of the cochlea.Similarly, channel 2 may be associated with bins of higher frequency andmay be dedicated to second and third electrodes 66. Also, channel 3 maybe associated with bins of even higher frequency and may be dedicated tothird and fourth electrodes 66. Likewise, channel N−1 may be associatedwith the highest frequency bins and may be dedicated to (N−1)^(st) andN_(th) electrodes 66, assuming the N^(th) electrode to be the one thatis closest to the basal end of the cochlea. Each channel may thereforedefine the relationship of a set of consecutive bins over an electrodepair. A virtual electrode may be stimulated by exciting an adjacent pairof bounding electrodes simultaneously while assigning different relativeweights applied to them, as exemplified in and discussed in connectionwith FIG. 2. The following discussion in connection with FIG. 5 furtherdetails this strategy.

FIG. 5 shows a collection of components that may be referred to assubsystem 500. Such a subsystem may be used to stimulate an optimalvirtual electrode using a pair of adjacent electrodes 66 for at leastone channel 44 of FIG. 4. Each set of frequency bins associated with achannel may be provided as input to subsystem 500. Accordingly,subsystem 500 may be applied to any channel. The processing circuitry incochlear system 30 of FIG. 1 may include several instances of subsystem500. For example, such processing circuitry includes N−1 iterations ofsubsystem 500 in order for all subsystem to be applied to all channelssimultaneously. Each pair of electrodes may however be stimulatedwithout stimulating any other electrodes in the array. Accordingly, inan alternative embodiment of the present invention, each pair ofelectrodes may be stimulated sequentially whereby each channel may beapplied to the same subsystem 500. In such an embodiment, only oneiteration of subsystem 500 would be included in the processingcircuitry.

Different groups of electrode pairs that are separated by a sufficientnumber of electrodes so as to avoid undesirable electrode interferencemay be stimulated simultaneously. This may increase the stimulationspeed by achieving an effective stimulation rate. For example, in asystem that uses 17 electrodes, hence 16 channels, four iterations ofsubsystem 500 may be used. In this case, there may be four multiplexedstimulation periods. In the first period, the first subsystem may beassociated with channel 1 and coupled to electrodes 1 and 2, the secondsubsystem may be associated with channel 2 and coupled to electrodes 5and 6, the third subsystem may be associated with channel 3 and coupledto electrodes 9 and 10 and the fourth subsystem may be associated withchannel 4 and coupled to electrodes 13 and 14. In the second period, thefirst subsystem may be associated with channel 5 and coupled toelectrodes 4 and 5, the second subsystem may be associated with channel6 and coupled to electrodes 8 and 9, the third subsystem may beassociated with channel 7 and coupled to electrodes 12 and 13 and thefourth subsystem may be associated with channel 8 and coupled toelectrodes 16 and 17. In the third period, the first subsystem may beassociated with channel 9 and coupled to electrodes 3 and 4, the secondsubsystem may be associated with channel 10 and coupled to electrodes 7and 8, the third subsystem may be associated with channel 11 and coupledto electrodes 11 and 12 and the fourth subsystem may be associated withchannel 12 and coupled to electrodes 15 and 16. Finally, in the fourthperiod, the first subsystem may be associated with channel 13 andcoupled to electrodes 2 and 3, the second subsystem may be associatedwith channel 14 and coupled to electrodes 6 and 7, the third subsystemmay be associated with channel 15 and coupled to electrodes 10 and 11and the fourth subsystem may be associated with channel 16 and coupledto electrodes 14 and 15. In such an arrangement, four groups ofelectrode pairs separated by at least two electrodes may be stimulatedin each stimulation period such that the entire array of 17 electrodesis stimulated in four consecutive periods using four iterations ofsubsystem 500 for 16 channels.

Subsystem 500 of FIG. 5 may include envelope detector 52, peak locator54, navigator 56 and mapper 58. The functions performed by these andother components, such as those shown in FIGS. 4, 9 and 12 mayalternatively be performed through software. Envelope detector 52 mayapply an envelope detection algorithm to a channel. For example, aHilbert transform may be applied to the input signal. The Hilberttransform may be computed using the real and imaginary parts of theinput signal. The envelope of the signal may then be computed based onthe Hilbert transform. The envelope may be a Hilbert envelope computedas the square root of the sum of the squared imaginary parts and thesquared real parts. Preferably, if i_(start) and i_(end) denote the binboundaries of each channel, H_(r)[τ,t] refers to the real part of theHilbert transform while H_(i)[τ,t] refers to the imaginary part of theHilbert transform and HE[t] refers to the computed envelope, then:

$\begin{matrix}{{H_{r}\left\lbrack {\tau,t} \right\rbrack} = {{\sum\limits_{k = i_{start}}^{i_{end}}\;{{F\left\lbrack {k,t} \right\rbrack} \cdot {\cos\left( {\frac{2\;\pi}{m}k\;\tau} \right)}}} - {{{IF}\left\lbrack {k,t} \right\rbrack} \cdot {\sin\left( {\frac{2\;\pi}{m}k\;\tau} \right)}}}} \\{{H_{i}\left\lbrack {\tau,t} \right\rbrack} = {{\sum\limits_{k = i_{starti}}^{i_{end}}\;{{F\left\lbrack {k,t} \right\rbrack} \cdot {\sin\left( {\frac{2\;\pi}{m}k\;\tau} \right)}}} + {{{IF}\left\lbrack {k,t} \right\rbrack} \cdot {\cos\left( {\frac{2\;\pi}{m}k\;\tau} \right)}}}} \\{{{HE}\lbrack t\rbrack} = \sqrt{{H_{i}\lbrack t\rbrack}^{2} + {H_{r}\lbrack t\rbrack}^{2}}}\end{matrix}$

The logarithm of envelope HE may be computed directly on the sum of thesquares in order to avoid computing the square root of the sum. Theenvelope update rate for the channel—i.e., the rate at which theenvelope should be computed—may correspond to the channel bandwidth. Thechannel bandwidth may be proportional to the number of bins in thechannel multiplied by the bin width. The bin width may be equal to thesampling rate r divided by the length m of the FFT. Preferably, theproportionality factor is the same as that used for over-sampling.Accordingly, if o is the over-sampling factor—e.g., the ratio betweenthe sampling rate r and the Nyquist rate—the envelope update rate for achannel may be equal to the number of bins in the channel multiplied byo.r/m.

In practice, computing the envelope for a channel may be computationallyprohibitive, especially for wide channels. To solve this problem,envelope detector 52 may implement process 600 illustrated in FIG. 6 sothat the envelope may be more easily computed for a channel. At step610, a channel may be broken into sub-channels. Each of the sub-channelsmay have a bandwidth for which envelope computation is not prohibitive.Preferably, each of the sub-channels may have a bandwidth that is lessthan or equal to the sampling rate r divided by the over-sampling factoro. At step 620, the envelope for each sub-channel is derived. Theenvelope may be derived by computing the Hilbert envelope for eachsub-channel, as described above. At step 630, the envelope for theentire channel is computed as the root-mean square of the envelopes ofthe sub-channels. More specifically, the envelope may be computed as thesquare root of the sum of the squared envelopes computed for eachsub-channel. Process 600 may be applied to one or more channels. Forexample, process 600 may apply to the widest channel, or alternatively,to a subset of channels for which traditional envelope computation maybe considered computationally challenging.

According to a preferred embodiment of the present invention, thestimulation frequency for a channel may be computed as the frequency atwhich the spectral peak is located—i.e., the frequency at which thesignal has the highest amplitude—in the channel. Peak locator 52 of FIG.5 may be used to calculate such a frequency. FIG. 7 illustrates aprocess 700 that may be implemented by peak locator 52 in order tocalculate such a frequency for a channel.

At step 710, the bin having the most energy in the channel isdetermined. That may be achieved by computing the energy in each bin inthe channel and comparing the computed bin energies. Bin energy may becomputed by taking the sum of the square of the real and imaginary partsof the input signal. At step 720, a determination is made as to whetherthe bin with the most energy corresponds to a local maximum over theentire spectrum. Whether the bin with the most energy is a local maximummay be determined by determining whether the computed energy for thatbin is larger or equal to the computed energies for adjacent bins.

If it is determined that the bin with the most energy is a local maximumat step 720, then the spectral peak location is calculated at step 730.The spectral peak location may be set to be the location of a pointwithin the bin that has the most energy or within neighboring bins. Sucha point may be located in the middle of the bin that has the mostenergy. In alternative and preferable embodiments of the presentinvention, the spectral peak location may be calculated by fitting afunction between a group of points that lie within the bin that has themost energy and within neighboring bins, and computing the location atwhich the maximum of the function exists. Such a function may be chosenbased on the window function w[j]. For example, in the situation wherethe window function is a weighted sum of a Hanning and a Blackmanwindow, the spectral peak location may be calculated by fitting aparabola between the amplitude of the bin that has the most energy andthe amplitudes of the two neighboring bins. Preferably, if i_(peak) isthe index of the bin corresponding to the local maximum and k refers tothe spectral peak location, then:

$k = {\frac{{{F\left\lbrack {i_{peak} + 1} \right\rbrack}}^{2} - {{F\left\lbrack {i_{peak} - 1} \right\rbrack}}^{2}}{2 \cdot \left( {{{F\left\lbrack {i_{peak} + 1} \right\rbrack}}^{2} + {{F\left\lbrack {i_{peak} - 1} \right\rbrack}}^{2} - {2 \cdot {{F\left\lbrack i_{peak} \right\rbrack}}^{2}}} \right)} + i_{peak}}$

More accurate results may be obtained by computing the logarithm of theamplitudes of the chosen bins, although such calculations may be morecomputationally intensive. For example, the spectral peak location maybe calculated as:

$k = {\frac{{\log{{F\left\lbrack {i_{peak} + 1} \right\rbrack}}} - {\log{{F\left\lbrack {i_{peak} - 1} \right\rbrack}}}}{2 \cdot \left( {{\log{{F\left\lbrack {i_{peak} + 1} \right\rbrack}}} + {\log{{F\left\lbrack {i_{peak} - 1} \right\rbrack}}} - {2\;{\log \cdot {{F\left\lbrack i_{peak} \right\rbrack}}}}} \right)} + i_{peak}}$

If it is determined that the bin with the most energy is not a localmaximum at step 720, then spectral peak location is estimated at step740. The spectral peak location k may be set to be the location of apoint that lies midway between that bin and whichever of the twoneighboring bins has the larger energy. This may be based on theapproximation that the spectral peak is located near the boundary of thechannel. In this situation, the spectral peak location may be estimatedfrom the following:

$k = \left\{ \begin{matrix}{i_{peak} - 0.5} & {{F\left\lfloor {i_{peak} - 1} \right\rfloor} > {F\left\lfloor i_{peak} \right\rfloor}} \\{i_{peak} + 0.5} & {{F\left\lbrack {i_{peak} + 1} \right\rbrack} > {F\left\lbrack i_{peak} \right\rbrack}}\end{matrix} \right.$

After the spectral peak location is obtained in either step 730 or step740, it is translated into frequency at step 750. The frequency at whichthe spectral peak is located within a channel may be computed bymultiplying the spectral peak location by the bin width r/m. Thecomputed stimulation frequency may correspond to the frequency at whichthe spectral peak is located. The Thus, if f_(s) refers to thestimulation frequency, then:f _(s) =k*r/m

Peak location may be computed at a rate that is proportional to the binwidth. Preferably, the proportionality factor is equal to theover-sampling factor o. Accordingly, peak locator 52 may be used tocompute the stimulation frequency at a rate of o.r/m.

After the stimulation frequency for a channel is calculated, such afrequency may be translated into a desired cochlear location—i.e., thecorresponding physical location along the electrode array that may beassociated with that frequency. This may be accomplished byinterpolating the cochlear location from a frequency-to-location table,which was mentioned above. More particularly, the frequency-to-locationtable may map each bin to a physical location along the electrode array.Preferably, if FtL[i] refers to function that performs such mapping and1 refers to the desired cochlear location corresponding to thestimulation frequency for a channel, then:

$l = {{F\; 2\;{L\left\lbrack i_{peak} \right\rbrack}} + \left\{ \begin{matrix}{\left( {{F\; 2\;{L\left\lbrack {i_{peak} + 1} \right\rbrack}} - {F\; 2\; L\left\lfloor i_{peak} \right\rfloor}} \right) \cdot \left( {k - i_{peak}} \right)} & {k > i_{peak}} \\{\left( {{F\; 2\;{L\left\lbrack i_{peak} \right\rbrack}} - {F\; 2\;{L\left\lbrack {i_{peak} - 1} \right\rbrack}}} \right) \cdot \left( {k - i_{peak}} \right)} & {k \leq i_{peak}}\end{matrix} \right.}$

A pair of adjacent electrodes may be stimulated using relative currentweights in order to stimulate the optimal virtual electrode at thedesired cochlear location corresponding to the computed stimulationfrequency for a channel. After the desired cochlear location isdetermined, navigator 56 of FIG. 5 may translate such a location intoweights assigned to a current that may be applied to each electrode inorder to stimulate the virtual electrode at the desired cochlearlocation. The weighted currents may be transmitted to electrode array 16through device 14 of FIG. 1.

More specifically, navigator 52 may compute weight w that may beassociated with the second electrode in an electrode pair, while thefirst electrode is associated with weight 1−w. Preferably, if l₀ refersto the nominal location of the first electrode, then:w=l−l ₀

Navigator 56 may also round the computed location to the nearestallowable cochlear location. In addition, navigator 56 may also ensurethat stimulation does not exceed the boundaries of the channel.Preferably, navigator 56 applies the following constraint:

$w = \left\{ \begin{matrix}0 & {w < 0} \\1 & {w > 1} \\w & {otherwise}\end{matrix} \right.$

Moreover, navigator 56 may ensure that, in the event that one of theelectrodes in a pair of electrodes to be stimulated is disabled, all ofthe current is applied to the other electrode. This may be useful duringthe fitting discussed below. Preferably, if E1 and E2 refer to the firstand second electrodes in an electrode pair, respectively, navigator 56may apply the following constraint:

$w = \left\{ \begin{matrix}0 & {E\; 2\mspace{14mu}{disabled}} \\1 & {E\; 1\mspace{14mu}{disabled}} \\w & {{all}\mspace{14mu}{channels}\mspace{14mu}{enabled}}\end{matrix} \right.$

Mapper 58 of FIG. 5 may receive the outputs of each of envelope detector52 and navigator 56. Mapper 58 may derive from these inputs streams ofpulse sets that may be applied to an electrode pair for a channel tostimulate that pair. More specifically, mapper 58 may modify a currentbased on the envelope computed by envelope detector 52 and split thecurrent between the electrode pair based on the relative weightscomputed by navigator 56 for a channel. Preferably, if M1 and M2 referto the mapping functions applied to the first and second electrodes inthe electrode pair for the channel, then currents I1 and I2 applied tothe first and second electrodes may be computed as follows:I2=M2(max(HE))·wI1=M1(max(HE))·(1−w)

max (HE) refers to the largest envelope value computed using envelopedetector 52 for the channel, in case the stimulation rate is slower thanthe envelope update rate. In case the stimulation rate is faster thanthe envelope update rate, the most recently computed envelope value mayused. Applying currents I1 and I2 simultaneously to electrodes E1 and E2respectively may stimulate a virtual electrode at a location along thecochlear array that corresponds to the frequency at which a spectralpeak is located within a particular channel. Envelope detection, peaklocation and weight computation may be applied to each channel in orderto stimulate all possible electrode pairs through currents derivedthrough one or more mappers across the entire sound spectrum for whichthe FFT is computed.

The above equations relating to the currents applied to electrodes E1and E2 may be particularly suitable if the electrical fields resultingfrom the stimulation of these electrodes interact strongly such that theloudness perceived by the user is appropriate. However, if such aninteraction is weaker at a location corresponding to the stimulationfrequency such that the perceived loudness is lower, the above equationsmay be modified so as to compensate for such loss in loudness. Morespecifically, I1 and I2 may be multiplied by a factor that increases asw increases from 0 to 0.5 or as w decreases from 1 to 0.5. Such a factormay be empirically determined based on user response to perceivedloudness. Such a factor may equal 1 near w=0 and w=1.

As mentioned above, it would be desirable to devise a method for fittingsound processing strategies such as ones that utilize simultaneousstimulation of electrodes. FIG. 8 illustrates process 800 for performingsuch fitting. Process 800 may be applied to one group of electrodes at atime.

At step 810, a group of electrodes known as a fitting group is selected.The fitting group preferably includes 4 or 5 electrodes but may includeany desired number of electrodes. For each electrode that is notincluded in the fitting group, a determination as to whether a partnerelectrode for that electrode exists in the fitting group is made at step820. A partner electrode is an electrode that would be stimulatedsimultaneously with the electrode in question—i.e., an electrode whoseelectric field would be superposed to that of the electrode in questionso as to stimulate a virtual electrode when the electrode in questionand its partner electrode are stimulated simultaneously. Such a partnerelectrode may typically be located relatively close to the electrode inquestion. A known sound stimulus to which the patient is comfortable maybe applied to the patient's cochlear system in order to help determinewhether a partner electrodes exist at step 820. The determination atstep 820 may be based on whether the electrodes are defined as partnerelectrodes or assigned to the same channel for simultaneous stimulation.Alternatively, the determination may be based on whether the user'sperception of sound changes when the partner electrode is enabled andthe stimulus is applied. For a stimulation strategy that utilizessimultaneous stimulation of pairs of adjacent electrodes, the electrodein the fitting group would be considered a partner electrode to theelectrode(s) adjacent to it.

If a partner electrode exists in the fitting group, then the portion ofthe current that would otherwise be applied to the electrode in questionis directed to the partner electrode at step 830. This may be madepossible by disabling all electrodes in the electrode array that are notpart of the fitting group because navigator 56 of FIG. 5 may ensure thatall of the current otherwise applied to disabled electrodes is insteadapplied to its partner electrode, as discussed above. This may berepeated for other fitting groups having at least another electrode asprocess 800 reverts back to step 810 from step 830 until all desiredfitting groups are selected. Process 800 may result in an accurateestimation of the current level required for each electrode because theloudness generated by splitting a current between at least one electrodeand its partner electrode(s) is approximately equal to the loudnessgenerated by applying the entire current to the at least one electrode.

FIG. 9 shows a collection of components that may be referred to assubsystem 900. Subsystem 900 may be an alternative embodiment of asubsystem that may be used to stimulate an optimal virtual electrodeusing a pair of adjacent electrodes 66 for at least one channel 44 ofFIG. 4. Like subsystem 500, subsystem 900 may be applied to eachchannel, whereby each set of frequency bins associated with a channelmay be provided as input to subsystem 900. The processing circuitry incochlear system 30 of FIG. 1 may include one or more instances ofsubsystem 900. For example, such processing circuitry may include asingle subsystem 900. As another example, the processing circuitry in acochlear system that that utilizes N electrodes may include N−1iterations of subsystem 900 in order for such a subsystem to be appliedto all channels simultaneously. Alternatively, such processing circuitrymay include any other number of iterations of subsystem 900 such thatdifferent groups of electrode pairs are stimulated simultaneously so asto achieve an effective pulse repetition rate. Such electrode pairs maybe separated by a sufficient number of electrodes so as to avoidundesirable electrode interference.

Like subsystem 500, subsystem 900 may include envelope detector 52, peaklocator 54 and navigator 56, which were discussed in connection withFIGS. 5-7. Envelope detector 52 may apply an envelope detectionalgorithm to a channel from which envelope HE may be computed. Peaklocator 54 may be used to locate the frequency at which the spectralpeak is located within a channel and therefore compute the correspondingcochlear location along the electrode array. Navigator 56 may translatesuch a location into weights w and (1−w) that may be assigned to acurrent applied to each electrode in order to stimulate the virtualelectrode at the desired cochlear location.

As mentioned above, it is desirable to present temporal information insound processing strategies such as ones that utilize simultaneousstimulation of electrodes. Carrier synthesizer 90, which may also beincluded in subsystem 900, may be used to derive carrier c in order topresent such temporal information. Carrier synthesis may be performed ata rate that is equal to the frame rate FR—i.e., the rate at whichindividual channels may be updated.

The carrier preferably has a frequency that corresponds, or isproportional, to the stimulation frequency. As discussed above, such afrequency may be the one at which the spectral peak within a channel islocated, as computed by peak locator 54. In an alternative embodiment ofthe present invention, such a frequency may be computed usingconventional methods. Such an embodiment is shown in FIG. 12 wherein acollection of components may be referred to as subsystem 1200. Likesubsystems 500 and 900, subsystem 1200 may include envelope detector 52,peak locator 54 and navigator 56 and carrier synthesizer 90. Subsystem1200 also includes frequency estimator 120 for estimating a frequencyfor the carrier. Frequency estimator may derive the carrier frequency bycalculating the instantaneous frequency along with phase angle andmagnitude values, as described in U.S. Pat. No. 6,480,820. For example,the instantaneous frequency may be calculated from the derivative of thephase.

In a preferred embodiment of the present invention, the carrier may bederived using Gated Max Rate (“GMR”) stimulation. In this process,carrier synthesizer 90 of FIG. 9 or 12 may produce a waveform having afrequency f_(o) that is preferably equal to the frequency computedthrough peak locator 54 or frequency estimator 120. Although it would bedesirable to utilize a half-wave rectified and filtered sinusoid as thecarrier waveform, it may be more practical to utilize a square wave. Thewaveform may have a modulation depth that depends on f_(o). For example,the waveform may have a modulation depth that decreases when f_(o) isincreased. FIG. 10 illustrates process 1000 which may be implemented bycarrier synthesizer 90 to reconstruct a rectified sinusoidal waveformusing GMR stimulation.

At step 1010, a carrier waveform is chosen. The carrier waveform may bechosen such that its modulation depth may be dependent on the frame rateFR. For example, the carrier waveform may be chosen such that itsmodulation depth decreases linearly with f_(o), when f_(o) lies within aparticular range. Preferably, if M(f) refers to the modulation depthfunction, then:

${M(f)} = \left\{ \begin{matrix}1 & {f \leq \frac{FR}{2}} \\{2 \cdot \left( {1 - \frac{f}{FR}} \right)} & {{FR} > f > \frac{FR}{2}} \\0 & {f \geq {FR}}\end{matrix} \right.$

A carrier phase variable ph is also defined for each channel at step1010. ph may range from 0 to FR−1. At step 1020, ph may be increasedduring each frame by the minimum of f_(o) and FR. A determination as towhether ph is greater than or equal to FR is made at step 1030. If ph isgreater than or equal to FR, then FR is subtracted from ph at step 1040.Otherwise, ph remains unchanged. At step 1050, the carrier c is computedas:c=1−s·M(f)where

$s = \left\{ \begin{matrix}1 & {{ph} < \frac{FR}{2}} \\0 & {otherwise}\end{matrix} \right.$

In an alternative embodiment of the present invention, the carrier maybe derived using Frequency-Modulation Stimulation (“FMS”). In thisprocess, carrier synthesizer 90 of FIG. 9 or 12 may produce a waveformhaving a frequency that is proportional to the frequency f_(o) computedthrough peak locator 54 or frequency estimator 120. FIG. 11 illustratesprocess 1100 which is similar to process 1000 of FIG. 10 and which maybe implemented by carrier synthesizer 90 to reconstruct a rectifiedsinusoidal waveform using FMS stimulation.

At step 1110, a carrier phase variable ph is defined for each channel.ph may range from 0 to FR*n−1. n may be set at 0.5 or at any otherappropriate value. At step 1020, ph may be increased during each frameby the minimum of f_(o) and FR. A determination as to whether ph isgreater than or equal to FR is made at step 1030. If ph is greater thanor equal to FR, then FR is subtracted from ph at step 1040. Otherwise,ph remains unchanged. At step 1150, the carrier c is computed as:

$c = \left\{ \begin{matrix}1 & {{ph} = {{ph} - {FR}}} \\0 & {otherwise}\end{matrix} \right.$

Subsystems 900 and 1200 of FIGS. 9 and 12 may also each include mapper98, which may receive the outputs of each of envelope detector 52,navigator 56 and carrier synthesizer 90. Mapper 98 may derive from theseinputs streams of pulse sets that may be applied to an electrode pairfor a channel to stimulate that pair. More specifically, mapper 98 maymodify a current based on the envelope and carrier derived by envelopedetector 52 and carrier synthesizer 90, respectively, and split thecurrent between the electrode pair based on the relative weightscomputed by navigator 56 for a channel. Preferably, if M1 and M2 referto the mapping functions applied to the first and second electrodes inthe electrode pair for the channel, then currents I1 and I2 applied tothe first and second electrodes may be as follows:I2=M2(max(HE))·c·wI1=M1(max(HE))·c·(1−w)

Again, max(HE) refers to the largest envelope value computed usingenvelope detector 52 for the channel, in case the stimulation rate isslower than the envelope update rate. In case the stimulation rate isfaster than the envelope update rate, the most recently computedenvelope value may used. Preferably, mapping function M1 or M2 may becombined with carrier c prior to applying weight (1−w) or w. If theenvelope has not been computed between the previous and the nextcomputation, then the previous envelope value may used. Also, currentsI1 and I2 may be modified in order to compensate for a decrease inperceived loudness at locations where electrical field interaction isrelatively weak.

Applying currents I1 and I2 simultaneously to electrodes E1 and E2respectively may present temporal information associated with theelectrical signal representative of the sound in addition to stimulatinga virtual electrode at a location along the cochlear array thatcorresponds to the frequency at which a spectral peak is located withina particular channel. Envelope detection, peak location, carriersynthesis and weight computation may be applied to each channel in orderto stimulate all possible electrode pairs through currents derivedthrough one or more mappers across the entire sound spectrum for whichthe FFT is computed.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention. Forexample, the number of channels into which FFT frequency bins may beorganized may be a multiple of (N−1) in a stimulation strategy that usesan N-electrode array. If the multiplication factor is referred to as p,then a number of p channels may define the relationship of sets ofconsecutive bins in these channels over an electrode pair. For example,if p=2, then a pair of channels may define the relationship of two setsof consecutive bins in these channels over an electrode pair.

One skilled in the art will appreciate that the present invention can bepracticed in other than the described embodiments, which are presentedfor purposes of illustration and not limitation, and the presentinvention is limited only by the claims which follow.

1. A method for stimulating a cochlea using a plurality of electrodesbased on a signal representative of sound, the method comprising:computing a frequency spectrum of the signal representative of sound;arranging the frequency spectrum into a plurality of channels such thateach channel corresponds to a range of frequencies that lie within thefrequency spectrum; computing a frequency at which a spectral peak islocated within the range of frequencies that corresponds to a channel ofthe plurality of channels; and stimulating a subset of the plurality ofelectrodes with a stream of pulse sets, the pulses in each set beingsimultaneously applied to the electrodes in the subset so as tostimulate a virtual electrode positioned at a location on the cochlea,the location on the cochlea corresponding to the frequency at which thespectral peak is located.
 2. The method of claim 1 further comprising:equalizing and compressing a dynamic range of the signal representativeof sound; buffering the signal representative of sound; and windowingthe buffered signal.
 3. The method of claim 1 wherein the computing thefrequency spectrum comprises applying an FFT algorithm to the signalrepresentative of sound in order to produce a representation of thesignal that is broken down into a plurality of frequency bins, thearranging the frequency spectrum comprises organizing the plurality ofbins into the plurality of channels.
 4. The method of claim 3 whereinthe computing the frequency at which a spectral peak is located withinthe range of frequencies that corresponds to the channel furthercomprises: determining a bin from the plurality of bins that has themost energy in the channel; determining whether the bin is a localmaximum over the computed frequency spectrum; and if the bin is a localmaximum, setting the frequency at which the spectral peak is located tocorrespond to a location of a point within the bin.
 5. The method ofclaim 4 further wherein the setting the frequency at which the spectralpeak is located comprises setting the frequency to correspond to alocation of a maximum of a function that is fit between a plurality ofpoints within the bin and two bins that are adjacent to the bin that hasthe most energy in the channel.
 6. The method of claim 4 furthercomprising: if the bin is not a local maximum, setting the frequency atwhich the spectral peak is located to correspond to a location of apoint that lies midway between the bin and another bin having the largerenergy of two bins that are adjacent to the bin that has the most energyin the channel.
 7. The method of claim 1 further comprising: assigning aplurality of weights, each weight corresponding to a portion of acurrent applied to an electrode in the subset so as to stimulate avirtual electrode positioned at the location on the cochleacorresponding to the frequency at which the spectral peak is located;and splitting the current between the electrodes in the subset based onthe plurality of weights.
 8. The method of claim 7 wherein the assigningthe plurality of weights further comprises: determining a cochlearlocation corresponding to the frequency at which the spectral peak islocated; and translating the cochlear location into the plurality ofweights based on locations of the electrodes in the subset.
 9. Themethod of claim 8 wherein the determining the cochlear locationcomprises interpolating the cochlear location from afrequency-to-location table.
 10. The method of claim 8 wherein thetranslating the cochlear location into the plurality of weightscomprises: ensuring that stimulation does not exceed the range offrequencies in the channel; and in the event that at least one of theelectrodes in the subset is disabled, applying a portion of the currentapplied to the at least one of the electrodes to another electrode inthe subset.
 11. The method of claim 7 further comprising: computing anenvelope for a portion of the frequency spectrum of the signalrepresentative of sound; and modifying the current based on the computedenvelope.
 12. The method of claim 7 further comprising: deriving awaveform having a frequency that is based on the frequency at which thespectral peak is located; and modifying the current based on the derivedwaveform.
 13. The method of claim 12 wherein the waveform frequencycorresponds to the frequency at which the spectral peak is located, thewaveform having a modulation depth that decreases as the frequency atwhich the spectral peak is located increases.
 14. The method of claim 12wherein the waveform frequency is proportional to the frequency at whichthe spectral peak is located.
 15. The method of claim 7 furthercomprising: calculating an instantaneous frequency within the range offrequencies that corresponds to the channel; deriving a waveform havinga frequency that is based on the instantaneous frequency; and modifyingthe current based on the derived waveform.
 16. The method of claim 15wherein the waveform frequency corresponds to the instantaneousfrequency, the waveform having has a modulation depth that decreases asthe instantaneous frequency increases.
 17. The method of claim 15wherein the waveform frequency is proportional to the instantaneousfrequency.
 18. The method of claim 1 further comprising, for each of theother plurality of channels, performing the computing the frequency atwhich a spectral peak is located and the stimulating the subset of theplurality of electrodes.
 19. A cochlear implant system for stimulating acochlea based on a signal representative of sound, the systemcomprising: sound processing circuitry adapted to: compute a frequencyspectrum of the signal representative of sound; arrange the frequencyspectrum into a plurality of channels such that each channel correspondsto a range of frequencies that lie within the frequency spectrum; andcompute a frequency at which a spectral peak is located within the rangeof frequencies that corresponds to a channel of the plurality ofchannels; and an electrode array inserted into the cochlea and coupledto the sound processing circuitry, the array comprising a plurality ofelectrodes of which a subset is stimulated with a stream of pulse sets,the pulses in each set being simultaneously applied to the electrodes inthe subset so as to stimulate a virtual electrode positioned at alocation on the cochlea, the location on the cochlea corresponding tothe frequency at which the spectral peak is located.
 20. The cochlearimplant system of claim 19 further comprising: a microphone coupled tothe sound processing circuitry; a power source coupled to the soundprocessing circuitry; telemetry transmitter circuitry coupled to thesound processing circuitry for transmitting signals corresponding to thesignal representative of sound; an implantable receiver circuit forreceiving the corresponding signals and generating stimulation currents;and a cochlear stimulator coupled to the implantable receiver circuitand the electrode array, the cochlear stimulator for stimulating theelectrode array.
 21. The cochlear implant system of claim 19 furthercomprising: an automatic gain control circuit for equalizing andcompressing a dynamic range of the signal representative of sound; and abuffer coupled to the automatic gain control circuit and the soundprocessing circuitry.
 22. The cochlear implant system of claim 19wherein the sound processing circuitry comprises FFT circuitry forcomputing the frequency spectrum of the signal representative of soundin order to produce a representation of the signal that is broken downinto a plurality of frequency bins that are organized into the pluralityof channels.
 23. The cochlear implant system of claim 22 furthercomprising a peak locator for computing the frequency at which thespectral peak is located within the range of frequencies thatcorresponds to the channel.
 24. The cochlear implant system of claim 23wherein the peak locator computes the frequency at which the spectralpeak is located within the range of frequencies that correspond to thechannel by: determining a bin from the plurality of bins that has themost energy in the channel; determining whether the bin is a localmaximum over the computed frequency spectrum; and if the bin is a localmaximum, setting the frequency at which the spectral peak is located tocorrespond to a location of a point within the bin.
 25. The cochlearimplant system of claim 24 wherein the location of the point with thebin corresponds to a location of a maximum of a function that is fitbetween a plurality of points within the bin and two bins that areadjacent to the bin that has the most energy in the channel.
 26. Thecochlear implant system of claim 24 wherein if the bin is not a localmaximum, setting the frequency at which the spectral peak is located tobe a frequency that corresponds to a location of a point that liesmidway between the bin and another bin having the larger energy of twobins that are adjacent to the bin that has the most energy in thechannel.
 27. The cochlear implant system of claim 19 further comprising:a peak locator for computing the frequency at which the spectral peak islocated; a navigator coupled to the peak locator for assigning aplurality of weights, each weight corresponding to a portion of acurrent applied to an electrode in the subset so as to stimulate avirtual electrode positioned at the location on the cochleacorresponding to the frequency at which the spectral peak is located;and a mapper coupled to the peak locator, the mapper being configured tosplit the current between the electrodes in the subset based on theplurality of weights.
 28. The cochlear implant system of claim 27further comprising an envelope detector coupled to the mapper forcomputing an envelope for the channel, the mapper being furtherconfigured to modify the current based on the computed envelope.
 29. Thecochlear implant system of claim 27 further comprising a carriersynthesizer coupled to the mapper for deriving a waveform having afrequency that is based on the frequency at which the spectral peak islocated, the mapper being further configured to modify the current basedon the derived waveform.
 30. The cochlear implant system of claim 29wherein the waveform frequency corresponds to the frequency at which thespectral peak is located, the waveform having a modulation depth thatdecreases as the frequency at which the spectral peak is locatedincreases.
 31. The cochlear implant system of claim 29 wherein thewaveform frequency is proportional to the frequency at which thespectral peak is located.
 32. The cochlear implant system of claim 27further comprising: a frequency estimator for calculating aninstantaneous frequency within the range of frequencies that correspondsto the channel; and a carrier synthesizer coupled to the frequencyestimator and the mapper for deriving a waveform having a frequency thatis based on the instantaneous frequency, the mapper being furtherconfigured to modify the current based on the derived waveform.
 33. Thecochlear implant system of claim 32 wherein the waveform frequencycorresponds to the instantaneous frequency, the waveform having has amodulation depth that decreases as the instantaneous frequencyincreases.
 34. The cochlear implant system of claim 33 wherein thewaveform frequency is proportional to the instantaneous frequency.
 35. Amethod for use in a cochlear implant system, the method comprising:applying an FFT algorithm to a signal representative of sound in orderto produce a representation of the signal that is broken down into aplurality of frequency bins that are organized into a plurality ofchannels such that each channel comprises a subset of the plurality ofbins; determining a bin that has the most energy from a subset of theplurality of bins in a channel; determining whether the energy in thebin is larger than energies in other bins adjacent to the bin; if theenergy in the bin is larger than energies in the adjacent bins,computing a stimulation frequency to be a frequency that corresponds toa location of a maximum of a parabola that is fit between a plurality ofpoints within the bin and two of the adjacent bins; and if the energy inthe bin is not larger than energies in the adjacent bins, computing thestimulation frequency to correspond to a location of a point that liesmidway between the bin and another bin having the larger energy of theadjacent bins.
 36. A sound processing unit for use in a cochlear implantsystem, the unit comprising: circuitry for applying an FFT algorithm toa signal representative of sound in order to produce a representation ofthe signal that is broken down into a plurality of frequency bins thatare organized into a plurality of channels such that each channelcomprises a subset of the plurality of bins; and circuitry being adaptedto: determine a bin that has the most energy from a subset of theplurality of bins in a channel; determine whether the energy in the binis larger than energies in other bins adjacent to the bin; if the energyin the bin is larger than energies in the adjacent bins, compute astimulation frequency that corresponds to a location of a maximum of aparabola that is fit between a plurality of points within the bin andtwo of the adjacent bins; and if the energy in the bin is not largerthan energies in the adjacent bins, compute the stimulation frequency tocorrespond to a location of a point that lies midway between the bin andanother bin having the larger energy of the adjacent bins.